1. Technical Field
This invention relates to sound absorbing materials having excellent sound-absorbing characteristics.
2. Background Art
A host of sound absorbing materials are used to control noise in various locations such as music halls, gymnasiums, construction sites and tunnels. The sound absorbing materials used for noise control purposes are versatile and include felts such as glass wool, rock wool and sponge, soft porous materials such as foamed resins comprising open cells, membranous materials such as vinyl sheets, porous boards such as soft fibrous boards, porous sintered boards, metal fiber boards and foamed metal boards, and perforated plates such as punched metals. These sound absorbing materials have their own merits and demerits in terms of sound-absorbing characteristics, weatherability, cost and fabrication method and suitable types are used in accordance with the specific use of interest.
In order to attain improved overall performance including sound-absorbing characteristics, composite systems in which two or more of the sound absorbing materials described are combined by lamination or some other suitable methods have been used commercially. Some of these composite sound absorbing materials exhibit satisfactory performance in sound-absorbing characteristics and weatherability in certain applications.
One major disadvantage of the prior art sound absorbing materials is that the frequency bands in which they exhibit satisfactory sound absorbing characteristics are narrow and that the absorption coefficients that are actually achieved are by no means satisfactory.
With the recent demand for improving the living conditions of people, there has been a growing need for reducing the level of noise exposure both indoors and outdoors. Under these circumstances, it is desired to develop sound absorbing materials having excellent sound-absorbing characteristics.